The short answer
Read a marathon race day weather forecast for your finish time, not the start. Find the hours of your halfway and last 10 km (6.2 miles), then check heat stress (WBGT), dew point and feels like. At Berlin in 2021, a 3:00 runner met Moderate heat stress from km 12; a 5:00 runner met High at km 40.
Key takeaways
- The start-line forecast describes your first few kilometres only; slower runners race into warmer hours.
- Plan around the hottest stretch you'll run, usually the last 10 km.
- WBGT and dew point say more about race-day heat than temperature alone.
- Organisers' heat flags and medical advice override any forecast you read.
On this page
- How do you read a marathon race day weather forecast?
- Why does the weather change between your start and your finish?
- Which numbers in the forecast matter most on race morning?
- How does your finish time change your race weather?
- What should you change on race day if the forecast is hot?
- When should you check the forecast before a marathon?
- Questions runners ask
- Sources and method
Words this guide uses
- Pace-timed forecast
- The weather for each stretch of your route at the time you'll run it, from your start time and pace.
- Event Alert System
- The Chicago Marathon's four-level race-day warning, from green (Low) to black (Extreme), with actions for runners at each level.
How do you read a marathon race day weather forecast?
Read the marathon race day weather forecast for the hours you'll be running, stretch by stretch, not for the start line. Find when you'll pass halfway and when you'll run the last 10 km (6.2 miles), then read the heat stress, dew point and feels like for those hours.
That is a pace-timed forecast: the weather for each stretch of your route at the time you'll run it. It's the core idea of our running weather forecast guide, and it matters most on race day, when a few degrees decide your pace.
A 3:00 runner and a 5:00 runner start together but finish two hours apart. By noon, heat stress has often moved up a level. The forecast you saw for the 09:00 start describes neither runner's last 10 km.
Why does the weather change between your start and your finish?
Because most marathons start in the morning and run into the warmest part of the day. The slower your finish time, the more of that warming you meet.
Berlin's 2021 race, on 26 September, shows it clearly. Start air was 16.5°C (62°F) at 09:15 for everyone. These are three even-paced runners, from RunFast's ERA5 race history.
| Finish (pace per km, per mile) | Warmest air, °C (°F) | Hottest feels like, °C (°F) | Highest dew point, °C (°F) | Heat stress (WBGT) |
|---|---|---|---|---|
| 3:00 (4:16, 6:52) | 22.1 (72) | 25.4 (78) | 14.9 (59) | Moderate from km 12 |
| 4:00 (5:41, 9:09) | 23.4 (74) | 27.9 (82) | 14.9 (59) | Moderate from km 9 |
| 5:00 (7:07, 11:27) | 24.1 (75) | 29.1 (84) | 15.0 (59) | Moderate from km 7, High from km 40 |
The 5:00 runner spent more time in the heat and met more of it. Their last two kilometres reached High heat stress, a WBGT of 22.9°C (73°F). The 3:00 runner was finished before that hour came.
Which numbers in the forecast matter most on race morning?
Heat stress (WBGT) and dew point matter most on a warm day; feels like when wet matters most on a cold one. Air temperature on its own can mislead in both directions.
- WBGT combines air, humidity, sun and wind into one heat-stress number. RunFast grades it Low, Moderate from 18.3°C (65°F), High from 22.8°C (73°F) and Extreme from 27.8°C (82°F).
- Dew point tells you how well your sweat will evaporate. From 15°C (59°F) the air feels sticky; from 18°C (64°F) it's hard work.
- Feels like at your pace shows how the air feels on your skin, with the sun and your own airflow.
- Rain and wind decide whether a cool day turns dangerous.
Boston's 2018 race, on 16 April, is the cold example. Start air was 3.2°C (38°F), with 8.6 mm (0.34 in) of rain for a 3:00 runner and 16 mm (0.63 in) for a 5:00 runner. The feels like when wet was −3.6°C (26°F). The temperature looked chilly; the rain made it hazardous.
How does your finish time change your race weather?
A later finish means warmer hours and more sun, so slower runners usually face more heat. Ely and colleagues (2007) found that slower runners lost more time than the leaders as WBGT rose.
Boston's 2024 race, on 15 April, shows how early the heat can arrive. Start air was 14.9°C (59°F). Heat stress reached Moderate from km 6 for a 3:00 runner and from km 4 for a 4:00 runner. The 4:00 runner's hottest feels like was 28.4°C (83°F), in sunshine.
London's 2018 race, on 22 April, was Moderate from the start line for every finish time, with start air of 18.9°C (66°F). Some hot races start warm and stay warm.
What should you change on race day if the forecast is hot?
Slow your start, plan your drinks, and dress for the hottest stretch you'll run. Then follow the organisers' flags, which override any plan.
- Pace. Start slower than goal pace. Heat stress usually rises through the race, so the second half needs the reserve.
- Drinks. Know where the drink stations are and plan what you'll take at each. Agree the plan with your doctor if you have a medical condition.
- Kit. Light, loose clothing, a cap and sunglasses for a sunny late finish.
- Start position. If you can choose your wave, an earlier start gives you cooler hours.
Heat illness can strike fit runners. The American College of Sports Medicine's 2007 position stand says exertional heatstroke can affect seemingly healthy athletes even when it seems relatively cool. Early recognition and rapid cooling are what save lives. The Chicago Marathon's Event Alert System runs from green to black, with actions at each level. This guide is not medical advice.
When should you check the forecast before a marathon?
Look at race-day history weeks out, then check the forecast at ten days, at five, and daily from two days before. Act on the last two checks.
Ten days out, the forecast shows a warm or cool spell, not your hour at km 35. Our guide to trusting a marathon forecast explains why. If you have your own course file, the weather along your running route guide shows how to load it. For the history, our marathon weather history hub covers ten years at four great races.
Questions runners ask
What time should I look at in the marathon forecast?
Look at three times: your start, the hour you expect to pass halfway, and the hour of your last 10 km (6.2 miles). For a 4:00 marathon starting at 09:00, that means 09:00, about 11:00, and from about 12:00 to 13:00. The last of these is usually the warmest and matters most for pacing, especially on a sunny day.
Is a marathon forecast for the start line enough?
Not for most runners. The start-line forecast covers your first half hour. On Boston's 2024 race day, start air was about 15°C (59°F) and a 4:00 runner met a feels like of 28.4°C (83°F) later in the race, with Moderate heat stress from km 4. Check the hours you'll actually be running.
How should I change my pace if race day is hot?
Start slower than your goal pace and hold back in the first half, because heat stress usually rises through the race. Ely and colleagues (2007) found marathon times slow progressively as WBGT climbs from 5 to 25°C (41 to 77°F), and slower runners lose the most. Follow the organisers' flags and medical team on the day.
What should I wear for a cold, wet marathon?
Dress for the feels like when wet, not the air. Boston's 2018 race started at 3.2°C (38°F) in cold rain, and the feels like for runners dropped below freezing. A throwaway layer, a cap, gloves and a light shell you can carry help. Keep a dry layer at the finish, because you cool fast once you stop.
Does the London Marathon use heat flags?
We haven't found an official London Marathon page that sets out a heat-flag system, so we don't claim one. Read the organisers' race-week emails and the information at the start for the current safety rules. The Chicago Marathon publishes its four-level Event Alert System, which is the best-documented system of this kind among the big-city marathons.
See the heat at your pace.
Drop in your route (GPX, FIT or TCX), pick the day and your pace, and see the feels like, dew point and heat stress for every kilometre, timed to when you’ll be there. Free, in your browser, no sign-up.
Sources and method
- Medicine & Science in Sports & Exercise (American College of Sports Medicine) (2007). Impact of weather on marathon-running performance (Ely, Cheuvront, Roberts and Montain)
- Medicine & Science in Sports & Exercise (American College of Sports Medicine) (2007). American College of Sports Medicine position stand: exertional heat illness during training and competition (Armstrong et al.)
- Bank of America Chicago Marathon (2026). Rules and safety (Event Alert System)
- Journal of Occupational and Environmental Hygiene (2008). Modeling the wet bulb globe temperature using standard meteorological measurements (Liljegren et al.)
- Copernicus Climate Change Service (ECMWF) (2026). ERA5 hourly data on single levels from 1940 to present
Race-day figures come from RunFast's race history: hourly ERA5 reanalysis (Copernicus Climate Change Service, ECMWF) at grid points along each course, timed to the official start and an even-paced 3-, 4- or 5-hour finish, and run through the RunFast engine (dew point by the Magnus formula, WBGT by Liljegren et al. 2008, feels like at running pace by wind chill when cool and Steadman's apparent temperature with sun when warm). "Moderate from km 12" is the first stretch where WBGT reached 18.3°C (65°F); High starts at 22.8°C (73°F). Reanalysis is a modelled grid, not a thermometer at the start line. Clock times assume even pacing. Imperial values are conversions.
Wes Thomson
Wes Thomson builds RunFast, which times the heat, humidity and dew point to a runner's own pace along the route, and its sister Windline, live wind forecasts for cyclists. Both run on the same forecast engine. He writes these guides with one focus: what the weather will do to your run, kilometre by kilometre, and how to plan around it.